Tissue products that are strong, soft and flexible are desired by consumers. One way of obtaining a soft tissue product is to increase the amount of debonder in the tissue to reduce the level of hydrogen bonding between fibers. While this increases the softness of the tissue, it also makes the tissue very weak. On the other hand, increasing the strength of the tissue by increasing the level of refining or increasing the amount of chemical strength agents will increase the level of hydrogen bonding between fibers and increase stiffness, which is also undesirable since increased stiffness generally reduces softness. One way to avoid this dilemma is to apply a polymeric binder having a low glass transition temperature, and therefore a flexible backbone, to the outside surfaces of the sheet. Hydrogen bonds, which impart strength to the tissue but make the tissue stiff, are replaced with the more flexible bonds of the polymeric binders. Bonding that occurs is due primarily to van der Waals' attractive forces between the polymer molecules and between cellulose fibers and the polymer molecules. In some cases, the binder may include small amounts of crosslinking components capable of forming covalent bonds between polymer molecules as well as between polymer molecules and fibers.
This approach has been used for heavyweight tissue products such as paper towels. For example, VIVA® Towels is a single-ply product that uses a topical application of a flexible strength agent in combination with creping often referred to as double recreping. The creped basesheet is heavily debonded, then printed on one side with a cross-linking polyethylenevinylacetate latex binder and recreped. The process is repeated for the other side of the sheet to form a very flexible and strong sheet with better softness than other sheets at equivalent strength. The resulting products have significantly preferred bulk softness over similar products made by more traditional methods such as conventional wet-pressing and throughdrying processes employing typical dry strengh and wet strength agents known in the art. While the bulk softness of such products is improved, the binder printed on the outside of the sheet provides a tacky feel that can be detrimental to products such as facial and bath tissue. For bath and facial tissue, surface softness is as important as bulk softness and the tacky feel of the binder can negatively affect the consumer's perception of surface softness.
Therefore, there is a need to improve the strength and bulk softness of lighter weight products such as facial tissue and bath tissue, without sacrificing surface softness.